Cellulose is one of the most common natural vegetative biomaterials and it has great industrial importance. The chemical structure of cellulose is composed of the D-glucopyranose monomers forming linear homopolysaccharides bonded by β-(1,4) glucosidic bonds. The number of monomeric units changes according to the plant species, it is around 10 thousand in woods, and around 15 thousand in cotton. Natural cellulose is present together with hemicellulose and lignin in plant structure. For this reason, lignin and hemicellulose should be removed in order to purify cellulose.
Hemicelluloses are also a kind of polysaccharide, and they are present in plants generally in ratio of 20% by weight1. Hemicelluloses are obtained by extracting vegetative biomaterials with alkali solutions. Hemicelluloses have random amorphous structure and comprise many kind of polysaccharides (For example: arabinoxilanes; (1→3) and (1→4)-β-glucans, xyloglucanes, pectins and gluco- and galacto-mannans2). Hemicelluloses comprise much less monomeric units (100-200 units) than celluloses, and they can easily be dissolved in alkaline solutions and eluted after separation of pectic components from plants. 1Fang J, Sun R, Tomkinson J. Isolation and characterization of hemicelluloses and cellulose from rye straw by alkaline peroxide extraction. Cellulose. 2000; 7(1):87-107.2Sun J, Sun X, Zhao H, Sun R. Isolation and characterization of cellulose from sugarcane bagasse. Polymer degradation and stability. 2004; 84(2):331-339.
Lignin is the second most common natural biomaterial. Lignins coexist with the cellulose. Functionally, they are present in the cell walls of the vascular plants and the stems of the woody plants. Lignins are quite hydrophobic contrary to the cellulose. Hemicelluloses are both hydrophilic and hydrophobic, therefore they provide compatibility between the cellulose and lignin. Lignin controls carrying water from cell wall and plant stem.
The hydrophobic structure of lignin protects the plant from enzymatic and chemical decomposition. For this reason, it makes the cellulose difficult to be reached by the chemicals or the enzymes. Furthermore, it gives a dark color to cellulose. Cellulose should as white as possible for paper industry, antimicrobial agents and enzyme inhibitors should be removed as much as possible for bioethanol production, and cellulose should be as pure as possible in production of cellulosic chemicals such as CMC (carboxymethyl cellulose). Because of all these reasons, the process of removing lignin (delignification) has great importance industrially.
Kraft (sulfite) process is one of the most important delignification processes especially in pulp and paper industry. In this method, concentrated NaOH and Na2S solutions (White liquor), high temperature, high pressure and suitable processing devices which are suitable for these hard conditions are used. The vegetative material (mostly wood) treated in these conditions is purified from lignin and hemicellulose, remaining is harmful and foul-smelling chemical mixture which is called as black liquor.
Today, new technologies are continuously sought due to environmental concerns and it is desired to obtain cellulose in more economical (operational and capital) conditions. Organosolv process is one of the new technologies realized as a product of this search. Organic solutions are used for dissolving and removing lignin and hemicellulose in Organosolv method. Generally, ethanol:water and acetone:water mixtures are used, and biomaterials are treated in these organic mixtures under high temperature (160-200° C.) and pressure. Then the biomaterial mixture is subjected to solid liquid separation and some part of the organic solvent is recovered from the liquid part. In Organosolv method, contrary to kraft process, foul-smelling waste chemicals (as waste) do not come out. Furthermore, the quality of the produced lignin has better quality relative to the one in kraft method. Apart from all these advantages, high temperature, pressure and using organic solvents in Organosolv method also increases capital and operating costs just like kraft method3. 3Macfarlane A L. Organosolv delignification of willow: Kinetics, recovery and use. 2009.
Alternative delignification methods remove only some part of lignin. Especially, in bioethanol production, cellulose should be separated from lignin and hemicellulose as much as possible so that the enzymes can affect cellulose. The higher the delignification rate is, the more the bioethanol production increases. It has been shown that some part of lignin can be removed by subjecting biomaterials to several pretreatments. Silverstein et al. (2007) pretreated cotton stalks with H2SO4, NaOH, and H2O2 at concentrations of 0.5%, 1%, and 2% (w/v), 1:10 solid:liquid ratio, at 90° C. and at 121° C. under 15 psi pressure for 30, 60, and 90 minutes residence times and compared delignification ratios and glucose production yields after enzymatic hydrolysis of cellulose. They found that 2% NaOH pretreatment at 121° C./15 psi for 90 minutes gave the highest delignification ratio (65.63%) and highest cellulose conversion (60.8%)4. 4Silverstein R A, Chen Y, Sharma-Shivappa R R, Boyette M D, Osborne J. A comparison of chemical pretreatment methods for improving saccharification of cotton stalks. Bioresource Technology. 2007; 98(16):3000-3011.
Heap leaching technology is a kind of hydrometallurgical process. Mineral stones or wastes which are crumbled and agglomerated in a heap are irrigated with various solutions, and the valuable metals such as copper or gold therein are separated in this method. During irrigation, the irrigation solution starts to solve the valuable metals while flowing towards the bottom of the heap, and it gradually gets saturated. When the solution reaches the bottom of the heap, it is transferred to the storage pool via drainage system. Then, mineral is separated from this solution if it reaches the adequate saturation. Then, the parameters of irrigation solution such as pH, chemical amount are adjusted and it is given to the irrigation again. This cycle is continued until most of the valuable metals in the mineral are separated. In heap leaching method, tens of thousands even hundreds of thousands of mineral can be processed in open air and in the field without requiring complex and expensive systems such as reactor, heat, pressure. For this reason, capital and operating expenses are much less relative to other conventional mineral separation methods. It is also possible to elongate mineral irrigation time since operating expenses are much smaller. Thus, gold and copper heaps can be irrigated for months at the heap leaching systems. Especially it is not economical to process low content mine and mine slags and wastes with other conventional methods, they become economical in heap leaching. In other words, some minerals that cannot be separated with other methods can be separated with heap leaching method.
Today, there are lots of experience about technical infrastructure of heap leaching especially in mining and metallurgy sector. Especially due to using acidic solutions (sulfuric acid) for separating copper, and using basic solutions (alkali cyanide) for separating gold, heap leaching system can be performed with both acidic and basic solutions. At the same time, it is possible to operate by giving microorganisms to the system and providing the medium where they will live. In heap bioleaching method, mostly a bacterium named Acidithiobacillus ferrooxidans is used. This bacterium can live in acidic conditions (acidophilic), grow at ambient temperatures (20-45° C.), and use mines such as pyrite (FeS2) and chalcopyrite (CuFeS2) for obtaining energy. They are naturally present in mines such as pyrite and chalcopyrite. In heap bioleaching method, this bacterium is used in iron containing mines such as pyrite and chalcopyrite, and irrigation is performed with solutions comprising ideal chemicals such that it can live in, and air is also given to the heap from the bottom via pipes. Acidithiobacillus ferrooxidans bacterium oxidizes bivalent iron ion to trivalent iron ion under these conditions. The formed iron ion dissolves the copper in chalcopyrite mine and passes it down to the irrigation solution5. In addition to the heap bioleaching technology comprising all positive aspects of the heap leaching technology, the presence of microorganisms significantly increases the reaction rates and efficiency. Furthermore, the increase in reaction rate and efficiency also allows mines, slags and wastes with lower content to be processed6. Heap bioleaching technology can also be used in gold mine heaps to remove iron which lowers gold yield by surrounding the gold so increasing cyanide consumption. Smith R, Misra M, Dubel J. Mineral bioprocessing and the future. Minerals Engineering. 1991; 4(7-11):1127-1141.6Donati E R, Sand W. Microbial processing of metal sulfides. Springer Verlag; 2007.
Even though heap leaching and bioleaching methods are commonly used in metallurgy and there is lots of experience, there is no study in the literature for adapting the technology to delignification.